Three-way catalysts, which simultaneously perform oxidation of carbon monoxide (CO) and hydrocarbons (HC) and reduction of nitrogen oxides (NOx) have been used in the prior art as exhaust gas purification catalysts for automobiles. Such catalysts that are widely known include catalysts comprising precious metals such as platinum (Pt), rhodium (Rh) or palladium (Pd) supported on porous oxide carriers such as alumina (Al2O3). For the action of a three-way catalyst to simultaneously and effectively accomplish purification of the three components CO, HC and NOx, it is important for the air/fuel ratio (A/F ratio) supplied to the automobile engine to be controlled to the vicinity of the theoretical air/fuel ratio (stoichiometric ratio).
However, since the actual air/fuel ratio fluctuates around the stoichiometric value from the rich (fuel-rich atmosphere) end to the lean (fuel-lean atmosphere) end, depending on the running conditions of the automobile as well as other factors, the exhaust gas atmosphere also fluctuates to the rich end or lean end. Consequently, it is not always possible to ensure high purification performance with a three-way catalyst alone. In order to absorb the fluctuation in oxygen concentration of exhaust gas to increase the exhaust gas purification power of three-way catalysts, materials such as ceria (CeO2), that have an oxygen storage capacity (OSC) that allows them to store oxygen when the exhaust gas has a high oxygen concentration and release oxygen when the exhaust gas has a low oxygen concentration, have come to be employed in exhaust gas purification catalysts.
In order to prevent sintering of the precious metals acting as the active species in such exhaust gas purification catalysts even when they are used at high temperatures (for example, about 1000° C.), it is important for the precious metals to be retained in a highly dispersed stated as fine particles on the carrier. It is generally known that rare earth oxides have strong affinity with precious metals supported thereon and therefore prevent sintering of the precious metals. However, rare earth oxides such as ceria have low heat resistance and themselves undergo sintering at high temperatures of 1000° C. and above, and as a result the precious metals supported on such rare earth oxides also undergo sintering. For purposes such as exhaust gas purification catalysts, therefore, it is necessary to improve the heat resistance of the rare earth oxides themselves, such as ceria.
Japanese Unexamined Patent Publication No. 2005-313024 (Patent document 1) describes an exhaust gas purification catalyst characterized as being a catalyst with a precious metal supported on a particulate carrier, the particulate carrier comprising a core section composed mainly of zirconia (ZrO2) and a shell section composed mainly of ceria (CeO2), and the CeO2 content of the particulate carrier being between 40 mol % and 65 mol %, and states that the catalyst simultaneously has improved heat resistance due to the ZrO2 and inhibited sintering of precious metals due to the CeO2.
Also, Japanese Unexamined Patent Publication No. 2005-254047 (Patent document 2) describes an exhaust gas purification catalyst that comprises metal oxide particles containing ceria and either or both alumina and silica, and a precious metal supported on the metal oxide particles, wherein the metal oxide particles have a core section with a larger molar total of alumina and silica than the molar content of ceria and a surface layer with a larger number of moles of ceria than the molar total of alumina and silica, and the publication states that this catalyst has improved heat resistance for an exhaust gas purification catalyst with OSC due to ceria, because of the core section which is rich in either or both alumina and silica.
Japanese Unexamined Patent Publication No. 2005-314134 (Patent document 3) describes metal oxide particles having a core section containing a relatively large amount of ceria-zirconia solid solution and a shell section containing a relatively large amount of a second metal oxide comprising ceria or zirconia, and states that using ceria as the second metal oxide can provide OSC due to the ceria-zirconia solid solution in the core section while preventing sintering of the precious metal by the ceria in the shell section.
Japanese Unexamined Patent Publication No. 2005-313028 (Patent document 4) describes an exhaust gas purification catalyst having rhodium supported on metal oxide particles, wherein the metal oxide particles comprise a core section containing a relatively large amount of ceria and a shell section containing a relatively large amount of zirconia, and states that the catalyst exhibits OSC by the ceria in the core section and high heat resistance by the zirconia in the shell section.
Finally, Japanese Unexamined Patent Publication HEI No. 9-255331 (Patent document 5) describes a process for production of nanometer-sized monodisperse rare earth oxide and complex oxide ultrafine particles, wherein aqueous solutions containing different solubilized rare earth ions and a precipitating agent solubilized in the same manner are mixed and reacted, the site of reaction being the interiors of surfactant-forming reversed micelles in a nonpolar organic solvent, and states that the process allows production of monodispersed rare earth oxide ultrafine particles and complex oxide ultrafine particles with very small mean particle sizes, without using a high pressure, high-temperature process.
In Patent documents 1-4, the sols of metal oxides that are to form the core section and shell section are sequentially aggregated utilizing the difference in their isoelectric points, to produce a core-shell structure comprising a core section composed mainly of a first metal oxide and a shell section composed mainly of a second metal oxide. However, the core-shell structures produced by such processes do not allow easy control of the shell section thicknesses on the nanolevel, and thus there is a need for improvement in order to achieve an effect of increasing the heat resistance of the core-shell structure resulting from the metal oxides composing the core sections.
Although Patent document 5 describes production of nanometer-sized monodisperse rare earth oxide and complex oxide ultrafine particles by a reversed micelle method, it neither mentions nor suggests production of complex oxide particles with a core-shell structure.
It is therefore an object of the present invention to provide a core-shell structure having a controlled shell section thickness on the nanolevel, and therefore improved properties, and especially heat resistance, resulting from the material of the core section, as well as a process for its production.
It is another object of the invention to provide an exhaust gas purification catalyst that comprises the core-shell structure.